This invention relates to electromagnetic switches which are used to close and open starting motor circuits for engines in automotive vehicles, and like circuits.
An electromagnetic switch of this kind has been proposed by Japanese Provisional Utility Model Publication (Kokai) No. 61-18556 (hereinafter referred to as "Publication A"), which comprises a casing formed of an electrically conductive magnetic material and adapted to be fixed to a mounting structure such as a vehicle body, a bobbin wound with a coil, a plunger fitted in the bobbin for movement therein in response to energization of the coil to move a movable contact member into or out of contact with a pair of fixed contact members, yokes arranged at opposite axial ends of the bobbin and in contact with the casing and an insulating plate member secured to an axial end of the plunger opposed to the movable contact member. The fixed contact members are rigidly fitted through a covering member, and each have a flat contact portion at its base end disposed for contact with the movable contact member, and a rod-shaped portion extending through the covering member and having an opposite outer end externally threaded for connection with a connecting member of an external circuit by means of a nut fitted on the threaded outer end.
According to the electromagnetic switch of Publication A, the aforementioned insulating member is interposed between the plunger and the movable contact member for the following reason: the movable and fixed contact members, which are usually formed of copper, are worn due to their repeated contacting actions to produce a considerable amount of fine chips or particles (particles of copper). These chips or particle are scattered and accumulated on peripheral parts, which causes leakage current to flow from the contact members to the mounting structure (vehicle body) through the plunger, yokes, and casing, particularly when the contacts are closed by the plunger receded into the bobbin due to energization of the coil. The insulating member serves to prevent the flow of leakage current.
However, the use of the insulating member does not only result in an increased number of component parts as well as in an increased number of man-hour for assemblage, but also involves the problem that the it is necessary to provide a supplement structure to obtain sufficient heat radiation of the hot contacts, especially the movable contact member, if an insulating member as aforementioned is employed.
Further, according to Publication A, if the electromagnetic switch is used to turn on and off an engine starting motor which consumes a large amount of current, the amount of fine chips or particles of copper produced is particularly large. The chips or particles are scattered and adhere to the movable contact member and the fixed contact members to degrade the insulation therebetween. To prevent this, a recess is formed in an upper end face of the plunger for collecting the chips or particles. However, since the chips or particles are not only scattered to the contact members, but also even to leader ends of the coil which are exposed to the outside of the coil, so that after a long term use the chips or particles may form an additional contact between the opposite leader ends of the coil.
Further, according to Publication A, the electromagnetic switch is not always used in a fashion being uprightly mounted on a mounting structure, but may be mounted in an inverted fashion, depending upon the conditions of the mounting space. However, when the electromagnetic switch is thus invertedly mounted, the above-mentioned additional contact will be formed, since the conventional switch has no suitable place on which the chips or particles are accumulated.
Another disadvantage with Publication A is that the fixed contact members have a special configuration having the flat contact portion and the rod-shaped portion. To obtain the special configuration, for example, a copper bar is wrought such that one end of the bar is squashed and stamped into a flat shape, and the other end threaded to form an external thread thereon, which takes much labor and time, resulting in a high manufacturing cost.
Another electromagnetic switch of this kind has been proposed by Japanese Provisional Utility Model Publication (Kokai) No. 61-16840 (hereinafter referred to as "Publication B"), which comprises a cylindrical bobbin, a coil wound around the bobbin, a plunger fitted in the bobbin for movement therein in response to energization of the coil, a movable contact member arranged for movement in response to the movement of the plunger, fixed contact members arranged for contact with the movable contact members, and a cylindrical casing accommodating the above-mentioned components and also serving as a magnetic path-forming yoke.
The electromagnetic switch according to Publication B essentially requires means for preventing rotation of the cylindrical bobbin relative to the cylindrical casing. Further, the electromagnetic switch is s constructed that when the coil is energized, the plunger moves downwardly into contact with the bottom surface of the casing whereby the movable contact member correspondingly moves into contact with the fixed contact members. Therefore, it also essentially requires means for preventing leakage current from flowing from the contacts to the casing through the plunger. Specifically, an insulating member as the leakage current-preventing means is interposed between the plunger and the movable contact member, or between the plunger and the casing.
Moreover, the electromagnetic switch according to Publication B structurally has limited resistance to vibrations. Therefore, in the case of mounting the switch in the body of an automotive vehicle, the switch has to be mounted at a location other than one where large vibrations may occur, thus being limited in mounting location.
Another electromagnetic switch of this kind has been proposed by Japanese Provisional Patent Publication (Kokai) No. 61-91820 (hereinafter referred to as "Publication C"), which carries a fuse unit formed of a fuse casing and a blade fuse accommodated therein, the fuse casing being secured on top of a main casing of the switch by means of engagement of engaging protuberances formed on opposite lateral side edges of a covering member covering the main casing, with respective engaging holes formed in opposite lateral side edges of the fuse casing. Further, the fuse casing in turn carries thereon a coupler for connecting the electromagnetic switch to connecting wires leading to load circuits. The coupler has a cover fitted over the fuse casing and engaged therewith by means of engagement of downwardly depending engaging pieces provided at opposite lateral side surfaces of the cover, with respective protuberances formed on opposite lateral side surfaces of the fuse casing.
The main casing has a pair of fixed contact members, one of which is to be connected to a power supply such as a car battery, and the other to a starting motor circuit, and a movable contact member being movable in response to movement of a plunger energized by a coil into and out of contact with the fixed contact members. The fuse casing has a coupling terminal formed integrally therewith and which couples the one fixed contact member connected to the power supply, to one end of the blade fuse, the other end of which is connected to load circuits which apply electrical loads on the automotive vehicle. The fuse casing also has another coupling terminal formed integrally therewith. The first-mentioned and another coupling terminals of the fuse casing are joined, respectively, to the one and the other fixed contact members of the main casing, by means of nuts threadedly fitted on externally threaded outer ends of the fixed contact members, to thus unite the fuse unit with the main casing of the electromagnetic switch.
According to Publication C, the coupling between the fuse casing and the cover of the coupler, and the coupling between the fuse casing and the covering member of the main casing are made by means of mere engagement of protuberances with engaging pieces, and that of protuberances with engaging pieces, respectively. Therefore, the assembly of the fuse unit and the electromagnetic switch is not perfectly resistant or proof against water. Moreover, since the two coupling terminals laterally project from the fuse casing, it is difficult to provide waterproof means for the assembly.
Further, according to Publication C, since the fuse casing of the fuse unit is formed integrally with the two coupling terminals, this integral formation requires a great number of man-hour to fabricate. Besides, the provision of the another coupling terminal on the fuse casing invites an increase in the manufacturing cost.